Very interesting question. I’ve not sculpted a toothless person before, but the main difference I’m seeing from image searches is that the lips seem to “suck backward” into the mouth cavity:

Which makes sense with no teeth for the lips to press against. Apparently DaVinci drew a fairly famous toothless man. I don’t see immediate evidence that missing teeth affects the jaw line directly…I’m thinking it could be more of a general old age thing when it comes to the jaw. I could be wrong though.